Gasification is a continuous thermal decomposition process in which solid organic or carbonaceous materials (feedstock) break down into a combustible gas mixture. The combustible gas components formed are primarily carbon monoxide (CO), hydrogen (H2), and methane (CH4). Other non-combustible gases such as nitrogen (N2), steam (H2O), and carbon dioxide (CO2) are also present in various quantities. The process of gasification involves pyrolysis followed by partial oxidation, which is controlled by injecting air or other oxygen containing gases into the partially pyrolysed feedstock. More specifically, biomass gasification is a sequence of reactions including water evaporation, lignin decomposition, cellulosic deflagration and carbon reduction. An external heat source begins the reaction, but partial oxidation provides heat to maintain the thermal decomposition of the feedstock. If concentrated oxygen is used, the resulting gas mixture is called syngas. If air (which includes nitrogen) is used as the oxidant, the resulting gas mixture is called producer gas. For simplicity, the term “Producer Gas” as used herein shall include both syngas and producer gas. Both gas mixtures are considered a “fuel gas” and can be used as a replacement for natural gas in many processes. They can also be used as a precursor to generate various industrial chemicals and motor fuels. When biomass is used as the feedstock, gasification and combustion of the Producer Gas is considered to be a source of renewable energy.
As a general matter, gasification offers a more efficient, cost effective and environmentally friendly alternative for extracting potential energy from solid feedstock as compared to combustion. As a result of gasification, the feedstock's potential energy can be converted to Producer Gas, which is cleaner burning, compressible and more portable. Producer Gas may be burned directly in some engines and burners, purified to produce methanol and hydrogen, or converted via the Fischer-Tropsch and other methods and processes into synthetic liquid fuel.
There are three common gasification processes: fluidized bed gasification, updraft gasification and downdraft gasification. The present invention is an improved downdraft gasifier. Therefore only a brief description of fluidized bed gasification and updraft gasification are provided and followed by a fuller discussion of current downdraft gasification.
Updraft Gasification
The counter-current fixed bed (“updraft”) gasifier consists of a fixed bed of feedstock on top of a large grate through which steam, oxygen and/or air flow upward. Updraft gasifiers typically require feedstock that is hardy and not prone to caking or clumping so that it will form a permeable bed. The updraft gasifier consists of a feedstock bed through which the oxidant (steam, oxygen and/or air) flows in from the bottom and exits through the top as gas. Updraft gasifiers are thermally efficient because the ascending gases pyrolyze and dry the incoming biomass, transferring heat so that the exiting Producer Gas is cooled when it exits the gasifier. However, significant amounts of tar are present in the Producer Gas, so it must be extensively cleaned before use, unless it is combusted at the point of generation. The tar can be recycled to the gasifier, but tar removal is complicated and costly. The updraft gasifier has been the standard of coal gasification for 150 years and it is also popular in biomass cooking stoves.
Fluidized-Bed Gasification
In a fluidized-bed gasifier, oxidant is blown through a bed of solid particles at a sufficient velocity to keep the solid particles in a state of suspension. The feedstock is introduced to the gasifier, very quickly mixed with the bed material and almost instantaneously heated to the bed temperature either externally or using a heat transfer medium. Most of these fluidized-bed gasifiers are equipped with an internal cyclone in order to minimize char (carried over into the Producer Gas stream) and remove fluidizing media from the Producer Gas. The major advantages include feedstock flexibility and the ability to easily control the reaction temperature, which allows for gasification of fine grained materials (sawdust, etc.) without the need of pre-processing. Fluidized-bed gasifiers also scale very well to large sizes. Unfortunately, problems with feeding, instability of the bed, build-up of residual carbon and ash sintering in the gas channels occur. Other drawbacks include high tar content of the Producer Gas (up to 500 mg/m3 gas), relatively low efficiency and poor response to load changes. Due to high operating and maintenance costs, this style of gasification is economically limited to large-scale applications, typically in excess of 100 tons per day.
Downdraft Gasification
In downdraft gasification, all feedstock, air and gases flow in the same direction—from top to bottom. Although updraft gasification is typically favored for processing of biomass feedstock and fluid bed gasification is typically used in gasification of coal, downdraft gasification process has a number of advantages. One advantage of downdraft gasification is low levels of tar in the resulting Producer Gas because the tars generated during pyrolysis must pass through the Oxidation Zone (defined below) and the char bed in the Reduction Zone (defined below) before exiting the gasifier. The high temperature of the Oxidation Zone and the top of the char bed breaks down the tars (i.e., thermal cracking). The result is a Producer Gas that may be cooled and more easily cleaned for use in reciprocating engines, gas-fired turbines and catalytic reforming processes.
Current downdraft gasification processes have some significant disadvantages that have prevented widespread adoption. These disadvantages are: (1) the feedstock generally must be pre-processed into standard sizes with similar chemical properties (without mixing different types of feedstock or different size pieces) to enable continuous gasification without bridging (i.e., jamming) the device or disrupting the quality of the Producer Gas; (2) the feedstock must have a standardized range of volatile components; (3) the feedstock must have a standardized calorific content (i.e., btu/lb); (4) generally, the gasifier must be stopped frequently for cleaning and removal of excess char that accumulates at the bottom of the gasifier; (5) the Producer Gas created is of inconsistent quality, and the gasifier is less productive and less efficient due to temperature changes caused by frequent shutdowns and variations in feedstock; (6) the gasifiers do not allow for reconfiguration during operation and must be shut down every time the oxidation reaction shifts from its designated location in the gasifier; (7) the gasifiers are not thermally stable over long periods of time and lose efficiency (or melt down); and (8) the gasifiers do not allow the location of the oxidation reaction to be moved in tandem with the reduction zone to compensate for different conditions required to gasify different types of feedstock and to generate different ratios of Producer Gas components. But the most significant disadvantage of current downdraft gasifiers is that (9) they require hearth loading such that the Oxidation Zone, also the hottest zone of the gasifier, be designed with a substantial restriction point (i.e., a restriction of approximately one half the diameter of the other sections of the gasifier).
In an ideal downdraft gasifier, there are three zones: a Pyrolysis Zone, an Oxidation Zone and a Reduction Zone (each defined below). In such an ideal gasifier, (1) the residence time of feedstock could be controlled in the Oxidation Zone (relative to the flow of feedstock through the rest of the gasifier) to allow the maximum amount of feedstock to undergo gasification before passing out of the Oxidation Zone into the Reduction Zone and (2) the Reduction Zone would be designed to cause the hot gas produced in the Oxidation Zone to mix with the char in the Reduction Zone as quickly and as thoroughly as possible to promote thorough gasification. Unfortunately, the restriction area in current gasifiers greatly impedes the overall volume of feedstock that can be moved through such a gasifier and disrupts the overall flow and output of Producer Gas.
The restriction areas found in prior art gasifiers are commonly referred to as the throat and hearth, which are an intentional design in current downdraft gasifiers as dictated by the prevailing theory, Superficial Velocity Theory.
Superficial Velocity (SV) is measured as:SV=Gas Production Rate/Cross Sectional Area=(m3/s)/(m2)=m/s
where s=time and m=distance.
Superficial Velocity Theory, when used to design downdraft gasifiers, dictates that a higher superficial gas velocity in the Oxidation Zone means a cleaner Producer Gas and less char by-product will be produced.
The physical restriction required by Superficial Velocity Theory in the Oxidation Zone itself limits both the entry and exit of feedstock in traditional downdraft gasifiers. It would be preferable to control the feedstock's velocity in the restriction area independent of its velocity throughout the rest of the gasifier in order to promote complete gasification and to reduce production of char by-product.
What is needed is a downdraft gasifier design that allows the flow rate of feedstock to be controlled as it passes through the Oxidation Zone with minimal restriction in order to improve the overall volume and flow of feedstock passing through the gasifier.